The hydrocarbons processed in reactor systems often have adverse secondary effects on the reactor metallurgy. Chemical attack on a metal substrate of the various components of reactor systems, such as furnace tubes, reactor vessels, or internal reactor structures may result in the degradative processes of carburization, metal dusting, halide stress corrosion cracking, and/or coking.
“Carburization” refers to the injection of carbon into the substrate of the various components of a reactor system. This carbon can then reside in the substrate at the grain boundaries. Carburization of the substrate can result in embrittlement, metal dusting, or a loss of the component's mechanical properties. “Metal dusting” results in a release of metal particulates from the surface of the substrate. “Coking” refers to a plurality of processes involving the decomposition of hydrocarbons to essentially elemental carbon. Halide stress corrosion cracking can occur when austenitic stainless steel contacts aqueous halide and represents a unique type of corrosion in which cracks propagate through the alloy. All of these degradative processes alone or in combination can result in considerable financial losses in terms of both productivity and equipment.
In the petrochemical industry, the hydrocarbons and impurities contained therein processed by hydrocarbon conversion systems can attack metal substrates associated with a hydrocarbon conversion system and the various internal reactor structures contained therein. “Hydrocarbon conversion systems” include isomerization systems, catalytic reforming systems, catalytic cracking systems, thermal cracking systems, and alkylation systems, among others.
“Catalytic reforming systems” refer to systems for the treatment of a hydrocarbon feed to provide an aromatics enriched product (i.e., a product whose aromatics content is greater than in the feed). Typically, one or more components of the hydrocarbon feed undergo one or more reforming reactions to produce aromatics. During catalytic reforming a hydrocarbon/hydrogen feed gas mixture is passed over a precious metal containing catalyst at elevated temperatures. Nonlimiting examples of catalysts useful for reforming include platinum and optionally rhenium or iridium on an alumina support, platinum on type X and Y zeolites, provided the reactants and products are sufficiently small to flow through the pores of the zeolites, platinum on cation exchanged type L zeolites and bimetallic catalysts. The bimetallic catalyst compositions employed in reforming operations include those comprising platinum, palladium or rhodium in combination with one or more metal promoters or metallic activating elements which form active catalyst complexes with a halogen promoter.
At elevated temperatures, the hydrocarbons and chemical reagents can react with the substrate of the reactor system components to form coke. In time, the coke can eventually break free from the substrate causing damage to downstream equipment and restricting flow at downstream screens, catalyst beds, treater beds, and exchangers. When the catalytic coke erupts from the surface of the substrate, then breaks free, a minute-sized piece of metal may be removed from the substrate to form a pit. Eventually, the pits will grow and erode the substrate of the hydrocarbon conversion system and internal reactor structures contained therein until repair or replacement is required.
Traditionally, the hydrocarbon feeds processed in catalytic reforming reactor systems contain small amounts of sulfur, which is an inhibitor of degradative processes, such as carburization, coking, and metal dusting. However, zeolitic reforming catalysts developed for use in catalytic reforming processes are susceptible to deactivation by sulfur. Thus, systems employing these catalysts must operate in a low-sulfur environment that offers less protection for the substrate metallurgy and increases the rate of degradative processes such as those discussed previously.
An alternative method for inhibiting degradation in a hydrocarbon conversion system, such as in a catalytic reforming reactor system, involves formation of a protective layer on the substrate surface with a protective material that is resistant to the degradative processes described above and chemical reagents. These protective materials form a layer termed a “metal protective layer” (MPL). Various metal protective layers and methods of applying the same are disclosed in U.S. Pat. Nos. 6,548,030, 5,406,014, 5,674,376, 5,676,821, 6,419,986, 6,551,660, 5,413,700, 5,593,571, 5,807,842, 5,849,969, and U.S. Patent Application Publication No. 2006/0275551A1, each of which is incorporated by reference herein in its entirety.
An MPL may be formed by applying a layer of a material containing at least one metal on a surface of the substrate to form an applied metal layer (AML). The AML may be thermally and/or chemically processed at elevated temperatures (“Cured”) as needed to form the MPL. The uniformity and thickness of the MPL, in addition to the composition of the MPL are important factors in its ability to inhibit reactor system degradation. While the MPL may provide protection of a substrate they may eventually require replacement or removal. For example, a partially degraded MPL may be removed before applying a new or different MPL or a reactor system may be converted to a new catalyst and new process conditions that could require removal of an existing MPL that may be incompatible with the new process conditions. The reactor system may have to be shutdown for some time period depending on the amount and nature of the MPL to be removed. Thus, it would be desirable to develop a methodology for efficiently removing a metal protective layer from a reactor surface.